Method for fabricating a semiconductor device

ABSTRACT

A method for fabricating a semiconductor device includes forming a first semiconductor layer on a front side of the semiconductor substrate. Additional semiconductor layers may be formed on a font side of the first semiconductor layer. The substrate is subsequently removed. In some embodiments, one or more additional semiconductor layers may be formed on the back side of the first semiconductor layer after the semiconductor substrate has been removed. Additionally, in some embodiments, a portion of the first semiconductor layer is removed along with the semiconductor substrate. In such embodiments, the first semiconductor layer is subsequently etched to a known thickness. Source regions and device electrodes may be then be formed.

RELATED APPLICATIONS

This patent application is a divisional of U.S. application Ser. No.11/338,043, entitled “Method For Fabricating A Semiconductor Device” andfiled Jan. 23, 2006, which claims the benefit of U.S. ProvisionalApplication No. 60/646,151, entitled “High-Voltage N-Channel InsulatedGate Bipolar Transistor (IGBT) in Silicon Carbide” and filed Jan. 21,2005. Each of the foregoing applications is expressly incorporatedherein by reference in its entirety.

GOVERNMENT RIGHTS

This invention was made with support from DARPA/MTO Grant No.N00014-02-1-0628; the government may have certain rights in thisinvention.

TECHNICAL FIELD

The present disclosure relates generally to methods for fabricatingsemiconductor devices, and more particularly to methods for fabricatinghigh-power semiconductor devices.

BACKGROUND

High-power semiconductor devices, such as insulated-gate bipolartransistors, are typically fabricated on thick substrates to, forexample, provide sufficient structural support for the semiconductordevice during the fabrication process. In some applications, the thicksubstrates may present a high parasitic series resistance in the devicedue to carrier freeze-out and/or low hole mobility in the substrate.

SUMMARY

A method for fabricating a semiconductor device may include forming asemiconductor substrate. The semiconductor substrate may be, forexample, a silicon-carbide semiconductor substrate. The method may alsoinclude forming a first semiconductor layer on a front side of thesemiconductor substrate. The first semiconductor layer may be so formedby, for example, epitaxially growing the first semiconductor layer. Thefirst semiconductor layer may be formed on either a silicon side or acarbon side of the semiconductor substrate. The first semiconductorlayer may be, for example, a drift semiconductor layer. The driftsemiconductor layer may have a first concentration of first typeimpurities that is less than a second concentration of first typeimpurities of the semiconductor substrate. Alternatively, thesemiconductor substrate may be doped with second type impurities. Thefirst semiconductor layer may be formed to a thickness of about onemicrometer. The method also includes removing the semiconductorsubstrate after the formation of the first semiconductor layer. Thesemiconductor layer may be so removed by using, for example, a chemicalmechanical polishing process. Removing the substrate may includeremoving a portion of the first semiconductor layer. The semiconductordevice may be an insulated-gate bipolar transistor. For example, thesemiconductor device may be a DMOS insulated-gate bipolar transistor ora UMOS insulated-gate bipolar transistor.

The method may also include forming a second semiconductor layer on afront side of the first semiconductor layer. The first semiconductorlayer may have a first concentration of first type impurities and thesecond semiconductor layer may have a second concentration of secondtype impurities. The method may also include forming a secondsemiconductor layer on a back side of the first semiconductor layerafter the semiconductor substrate has been removed. The firstsemiconductor layer may form a bottom semiconductor layer or a topsemiconductor layer of the device after the semiconductor substrate hasbeen removed. Removing the substrate may include removing a portion ofthe first semiconductor layer.

The method may further include determining a thickness of the firstsemiconductor layer subsequent to the removing step. The thickness ofthe first semiconductor layer may be determined, for example, by use ofa trench length measure process. For example, the thickness of the firstsemiconductor layer may be determined, subsequent to the removing step,based on a sheet resistivity of the first semiconductor layer.Determining the thickness of the first semiconductor layer may alsoinclude extrapolating a conductivity of the first semiconductor layer toa value of about zero. The first semiconductor layer may then be etchedto a known thickness based on the determined thickness.

A method for fabricating an insulated-gate bipolar transistor mayinclude forming a semiconductor substrate. The semiconductor substratemay be, for example, a silicon-carbide semiconductor substrate. Themethod may also include forming a drift semiconductor layer on a frontside of the semiconductor substrate. The drift semiconductor layer maybe so formed by, for example, epitaxially growing the driftsemiconductor layer on the semiconductor substrate. The driftsemiconductor layer may be formed on a silicon side or a carbon side ofthe semiconductor substrate. The method also includes forming a drainlayer on a front side of the drift semiconductor layer. The driftsemiconductor layer may be doped with first type impurities while thedrain semiconductor layer may be doped with second type impurities. Themethod may further include forming a first source region and a secondsource region in the drift semiconductor layer. The first and secondsource regions may be formed in the drift semiconductor layer subsequentto the removing step. The method may yet further include forming a firstsource contact on a front side of the first source region and a secondsource contact on a front side of the second source region. The methodmay include forming a gate oxide on a back side of the firstsemiconductor layer and a gate contact on a front side of the gateoxide. The method may also include forming a drain contact on a frontside of the drain semiconductor layer. The method may yet furtherinclude forming an additional semiconductor layer on a back side of thedrift semiconductor layer subsequent to the removing step.

A method for fabricating an insulated-gate bipolar transistor mayinclude forming a semiconductor substrate, which may be formed from asilicon-carbide material. The method may also include forming a firstsemiconductor layer on a front side of the first semiconductorsubstrate. The method may also include forming a second semiconductorlayer on a front side of the first semiconductor layer. The method mayyet further include removing the semiconductor substrate and a portionof the first semiconductor layer. The method may also include forming athird semiconductor layer on a front side of the second semiconductorlayer. The method may also include determining a thickness of the firstsemiconductor layer subsequent to the removing step. The thickness ofthe first semiconductor layer may be determined by, for example, use ofa trench length measure technique. Additionally, the thickness of thefirst semiconductor layer may be determined based on a sheet resistivityof the first semiconductor layer. Determining the thickness of the firstsemiconductor layer subsequently to the removing step may also includeextrapolating a conductivity of the first semiconductor layer to a valueof about zero.

A method for fabricating a semiconductor device on a semiconductorsubstrate may include forming a first semiconductor layer on a frontside of the semiconductor substrate. The method may also include forminga second semiconductor layer on a front side of the first semiconductorlayer. The first and the second semiconductor layers may be formed froma silicon-carbide material. The method may further include removing thesemiconductor substrate. Additionally, the method may yet furtherinclude processing the semiconductor device after the removing step toform an insulated-gate bipolar transistor.

The above and other features of the present disclosure, which alone orin any combination may comprise patentable subject matter, will becomeapparent from the following description and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the following figures,in which:

FIG. 1 is a flowchart of one embodiment of an algorithm for fabricatinga semiconductor device;

FIGS. 2 a-2 f are fragmentary, cross-sectional views of one embodimentof a semiconductor device at different stages of fabrication accordingto the algorithm of FIG. 1;

FIG. 3 is a flowchart of another embodiment of a sub-process of thealgorithm for fabricating a semiconductor device of FIG. 1;

FIGS. 4 a-4 e are fragmentary, cross-sectional views of one embodimentof a semiconductor device at different stages of fabrication accordingto the algorithm of FIGS. 1 and 3;

FIG. 5 is a flowchart of another embodiment of an algorithm forfabricating a semiconductor device;

FIG. 6 is a flowchart of one embodiment of an algorithm for determininga thickness of a semiconductor layer used in the algorithm of FIG. 5;

FIG. 7 is a fragmentary, cross-sectional view of one embodiment of asemiconductor device fabricated using the algorithm of FIG. 5;

FIGS. 8 a-8 b are views of a test area created during the execution ofthe algorithm of FIG. 6;

FIG. 9 is a graph showing a theoretical relationship between theconductivity and etching depth of a semiconductor wafer used in thealgorithm of FIG. 6;

FIG. 10 is a flowchart of another embodiment of an algorithm forfabricating a semiconductor device; and

FIG. 11 is a fragmentary, cross-sectional view of one embodiment of asemiconductor device fabricated using the algorithm of FIG. 10.

DETAILED DESCRIPTION OF THE DRAWINGS

While the concepts of the present disclosure are susceptible to variousmodifications and alternative forms, specific exemplary embodimentsthereof have been shown by way of example in the drawings and willherein be described in detail. It should be understood, however, thatthere is no intent to limit the concepts of the present disclosure tothe particular forms disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

Referring to FIG. 1, according to one embodiment, an algorithm 10 forfabricating a semiconductor device 50, such as an insulated-gate bipolartransistor device, includes a first step 12 in which a semiconductorsubstrate 52 is formed. The substrate 52 may be formed from anysemiconductor material and may be doped with N-type impurities or P-typeimpurities. For example, as illustrated in FIG. 2 a, in one particularembodiment, the substrate 52 is formed from silicon-carbide and doped toa concentration of “N+” with N-type impurities. However, it should beappreciated that the substrate 52 may alternatively be doped to aconcentration of “P+” with P-type impurities. The substrate 52 is formedto a thickness 54 sufficient to provide structural support to othersemiconductor layers during subsequent fabrication steps and/orfacilitate the removal thereof, as discussed in more detail below inregard to process step 18. For example, in one particular embodiment,the semiconductor substrate 52 is formed to a thickness 54 of about 350micrometers. However, other thicknesses 54 may be used in otherembodiments.

Once the substrate 52 is formed, a drift semiconductor layer 56 isformed on a front side 58 of the semiconductor substrate 52. It shouldbe appreciated, however, that one or more “buffer layers” may also beformed between the drift semiconductor layer 56 and the semiconductorsubstrate 52. As such, as used herein the term “formed on a front sideof the substrate/layer” is intended to mean formed on a top surface ofthe substrate/layer and/or formed on the front side of thesubstrate/layer with respect to the top surface of the substrate/layerwith a number of intervening “buffer layers”. Similarly, as used herein,the term “formed on a back side of the substrate/layer” is intended tomean formed on a bottom surface of the substrate/layer and/or formed onthe back side of the substrate/layer with respect to the bottom surfacewith a number of intervening “buffer layers”.

Illustratively, the drift layer 56 is formed from a silicon-carbidematerial, but in other embodiments, other types of semiconductormaterial may be used. The drift semiconductor layer 56 is doped to an“N−” concentration with N-type impurities. Additionally, the driftsemiconductor layer 56 is formed to a thickness 60, the value of whichis determined based on the particular application of the semiconductordevice 50. For example, in high-power applications, the blocking voltage(e.g., the drain-to-source voltage at which avalanche breakdown occurs)of the semiconductor device is dependent, in part, on the thickness ofthe drift semiconductor layer 56. As such, for high-power applications,the semiconductor device 10 may have a drift layer 56 of a greaterthickness than the drift layers of semiconductor devices for low powerapplications. For example, in one particular embodiment, the draftsemiconductor layer 56 is formed to have a thickness 60 of about 150micrometers to about 300 micrometers. The drift semiconductor layer 56may be formed on the front side 58 of the semiconductor substrate 52using any suitable semiconductor fabrication method. In one particularembodiment, the drift semiconductor layer 56 is formed on the top side58 of the semiconductor substrate via epitaxial growth.

In embodiments wherein the semiconductor substrate 52 is formed fromsilicon-carbide, the semiconductor substrate 52 has a silicon side and acarbon side due to the orientation of the silicon and carbon moleculesin the crystal lattice of the semiconductor wafer from which thesemiconductor substrate 52 is formed. That is, a silicon-carbide waferis sliced such that the surface of one side of the silicon-carbide waferis formed from substantially only silicon atoms and the surface of theopposite side of the silicon-carbide wafer is formed from substantiallyonly carbon atoms. Regardless, the drift semiconductor layer 56 may beformed on the semiconductor substrate 56 on the silicon side or thecarbon side. That is, the “front side” of the silicon-carbide wafer maybe determined to be the silicon side or the carbon side depending,typically, on the particular application. In one particular embodiment,the drift semiconductor layer 56 is formed on the silicon side of thesemiconductor substrate.

Referring back to FIG. 1, after the drift semiconductor layer 56 hasbeen formed, a drain semiconductor layer or anode layer 62 is formed inprocess step 16. The drain layer 62 is the semiconductor layer of thesemiconductor device 50 that will eventually be coupled with the drainelectrode, as discussed in more detail in regard to process step 26. Thedrain layer 62 is formed on a front side 64 of the drift semiconductorlayer 56. Similar to the illustrative drift layer 56, the illustrativedrain semiconductor layer 62 is formed from a silicon-carbide material.However, in other embodiments, other types of semiconductor material maybe used to form the drain semiconductor layer 62. In embodiments whereinthe drift layer is formed from a silicon-carbide material, the frontside 64 may be the silicon side or the carbon side of the driftsemiconductor layer 56. The drain semiconductor layer 62 may be formedby epitaxially growing the drain layer 62 on the front side of the draftsemiconductor layer 56. Alternatively, the drain semiconductor layer 62may be formed by ion implantation of the draft semiconductor layer 56.

The drain semiconductor layer 62 is illustratively doped to a “P+”concentration with P-type impurities. However, in other embodiments, thedrain semiconductor layer 62 may be doped to other concentrations ofother types of impurities. The drain semiconductor layer 56 is formed tohave a small thickness 66 with respect to semiconductor substrates oftypical high-power semiconductor devices such as typical high-powerinsulated-gate bipolar transistor device. Such a thin drain layer 62 mayreduce the parasitic series resistance of the semiconductor devicecompared to typical high-power semiconductor devices with thick drainlayers.

Referring back to FIG. 1, once the drain layer 62 has been formed inprocess step 16, the semiconductor substrate 52 is removed from thesemiconductor device 50 in process step 18. To do so, the semiconductordevice 50 may be flipped over such that the back side 53 of thesemiconductor substrate 52 is accessible. The semiconductor substrate 52may be removed from the semiconductor device 50 using any one of anumber of removal techniques. In one particular embodiment, thesemiconductor substrate 52 is removed from the semiconductor device 50using a chemical mechanical polish (CMP) process. As illustrated in FIG.2 d, once the semiconductor substrate 52 has been removed from thesemiconductor device 50, a back side 68 of the drift layer 56 isaccessible. The back side 68 is subsequently cleaned and any damage isrepaired using a suitable fabrication process. For example, in oneparticular embodiment, the back side 68 is cleaned and repaired using areactive ion etching (RIE) process and oxidation processes, if needed.

Referring back to FIG. 1, once the semiconductor substrate 52 has beenremoved from the semiconductor device 50, the fabrication of thesemiconductor device 50 is completed in process step 20. For example, inembodiments wherein the semiconductor device 50 is a DMOS insulated-gatebipolar transistor, the process step 20 includes a sub-step 22 whereindoped “P” wells or base regions 70, 72 are formed in the back side 68 ofthe drift semiconductor layer 56. It should be appreciated that althoughonly two doped wells are illustrated in FIG. 2 e, any number of dopedwells may be formed so as to facilitate the fabrication of any number ofsemiconductor devices. The doped “P” wells 70, 72 are doped with P-typeimpurities. In process step 24, the source regions 72, 74 and basecontact regions 76, 78 are formed in the doped wells 70, 72,respectively. The source regions 72, 74 are doped with an N-typeimpurity to an “N+” concentration. The base contact regions 76, 78 aredoped with a P-type impurity to a “P+” concentration.

Once the source regions 72, 74 and the base contact regions 76, 78 havebeen formed in the wells 70, 72, the gate, source, and drain electrodesof the semiconductor device 50 are formed in process step 26. To do so,as illustrated in FIG. 2 f, a gate dielectric layer 80, such as an oxidelayer, is formed on the back side 68 of the drift semiconductor layer56. A gate electrode 82 subsequently formed on the gate dielectric layer80. The gate electrode 82 is formed so as to cover the channel regions84 of the semiconductor device 50. Source electrodes 86, 88 are formedover the source regions 72, 74, respectively. Similarly, a drainelectrode 90 is formed on a front side 92 of the drain semiconductorlayer 52.

Although the illustrative semiconductor device 50 is a DMOS insulatedgate bipolar transistor, it should be appreciated that the algorithm 10may be used to fabricate other types of semiconductor devices such as,for example, a UMOS insulated gate bipolar transistor. To do so, theprocess steps 12-18 of algorithm 10 are performed in the mannerdescribed above in regard to each respective step to produce asemiconductor device similar to the semiconductor device illustrated inFIG. 2 d. However, to fabricate a UMOS insulated gate bipolar transistorusing the algorithm 10, additional sub-steps are executed in processstep 20 as illustrated in FIG. 3. As shown in FIG. 4 a, to fabricate aUMOS insulated gate bipolar transistor, in some embodiments, the processstep 20 begins with a sub-step 100 in which a current spreadingsemiconductor layer 112 is formed on the back side 68 of the driftsemiconductor layer 56. In the illustrative embodiment, the currentspreading semiconductor layer 112 is formed on the back side 68 of thedrift semiconductor layer by epitaxially growing (e.g., via chemicalvapor deposition) the current spreading layer 112 to a thickness 113.The current spreading layer 112 is illustratively formed fromsilicon-carbide and is doped with N-type impurities to an “N”concentration. The current spreading layer 112 may be so doped duringthe formation of the layer 20 or subsequently thereafter using anysuitable doping technique such as, for example, an ion implantation ordiffusion process. The current spreading layer 112 is doped to an N-typeimpurity concentration that is greater than the concentration of theN-type impurities of the drift layer 56.

Referring back to FIG. 3, once the current spreading layer 112 has beenformed in sub-step 100, a “P” well semiconductor layer or base region116 is formed on the current spreading layer 112 in process step 102. Asillustrated in FIG. 4 b, the “P” well semiconductor layer 116 is formedon a front side 114 of the current spreading layer. As discussed above,if the current spreading layer 112 is formed from a silicon-carbidematerial, the “P” well semiconductor layer 116 may be formed on eitherthe silicon side or the carbon side of the current spreading layer 112.The “P” well semiconductor layer 116 may be formed by epitaxiallygrowing the layer 116 on the current spreading layer 112 or,alternatively, the current spreading layer 112 may be formed to have anincreased thickness 113 and the “P” well semiconductor layer or baseregion 116 may be formed by incorporating P-type impurities into theupper region of the current spreading layer 112 by, for example, ionimplantation.

Referring back to FIG. 3, once the “P” well layer 116 has been formed, afirst source region 120 and a second source region 122 are formed in the“P” well semiconductor layer 116. The source regions 120, 122 areillustratively doped with N-type impurities to an “N+” concentration.The source regions 120, 122 may be formed in the “P” well semiconductorlayer by, for example, an ion implantation process. Once the sourceregions 120, 122 have been so formed, a gate trench 124 is formed in thesemiconductor device 110 in process step 106. As illustrated in FIG. 4d, the gate trench 124 is formed between the source regions 120, 122. Itshould be appreciated that, in some embodiments, the source regions 120,122 may be initially formed in sub-step 104 as a single, long sourceregion. Subsequently, in sub-step 106, the gate trench 124 is formedsuch that the single, long source region is divided into the sourceregions 120, 122. Alternatively, in some embodiments, the source regions102, 122 may be formed subsequent to the gate trench 124. Regardless,the gate trench 124 is etched down to the bottom side 68 of the driftlayer 56.

Referring back to FIG. 3, once the gate trench 124 has been formed inprocess step 106, the electrodes of the semiconductor device 110 areformed in process step 108. As illustrated in FIG. 4 e, a gatedielectric layer 126, such are an oxide layer, is formed in the gatetrench 124. A gate electrode 128 is subsequently formed on the gatedielectric layer 126 such that the gate electrode is formed over thechannel regions 136 of the semiconductor device 110. Source electrodes130, 132 are formed over the source regions 120, 122, respectively.Similarly, a drain electrode 134 is formed on a front side 92 of thedrain semiconductor layer 52.

It should be appreciated that although the algorithm 10 has beendescribed herein in reference to a DMOS insulated-gate bipolartransistor and a UMOS insulated-gate bipolar transistor, the algorithm10 may be used to form other types of high power semiconductor devices.For example, the algorithm 10 may be used to form a thyristor by theadditional of appropriate semiconductor layers. As such, the algorithmsdisclosed herein are not limited to the fabrication of insulated-gatebipolar transistors, but instead may be used to fabricate othersemiconductor devices for use in high power applications.

Referring now to FIG. 5, in another embodiment, an algorithm 150 forfabricating a semiconductor device, such as an insulated-gate bipolartransistor device, begins with a process step 152. In process step 152,a semiconductor substrate 202 is formed as shown in FIG. 7. Process step152 is substantially similar to the process step 12 of algorithm 10 andthe semiconductor substrate 202 may be substantially similar to thesemiconductor substrate 52 formed in process step 12. For example, thesemiconductor substrate may be formed from silicon-carbide. It should beappreciated that because the substrates 52, 202 are subsequently removedduring the fabrication of the semiconductor devices 50, 200, thesubstrates 52, 202 may be doped with any type of impurity. For example,the semiconductor 202 may be doped with N-type impurities to an “N+”concentration or with P-type impurities to a “P+” concentration.

Subsequently, in process step 154, additional semiconductor layers areformed on a front side 206 of the semiconductor substrate 202. Asillustrated in FIG. 7, the semiconductor layers are formed in atop-to-bottom sequence. That is, the semiconductor layer that will bethe top-most semiconductor layer when the semiconductor device iscomplete is formed first on the front side 206 of the substrate 202. Forexample, in embodiments wherein the semiconductor device 200 is a UMOSinsulated-gate bipolar transistor, an “N+” source semiconductor layer204 is formed on the front side 206 of the semiconductor substrate 202.In embodiments wherein the semiconductor device 200 is a DMOSinsulated-gate bipolar transistor, an “N−” drift layer may be formed onthe front side 206 of the substrate 202 instead. As such, it should beappreciated that the algorithm 150 may be used to fabricate a number ofdifferent semiconductor devices including, but not limited to, UMOSIGBTs, DMOS IGBTs, and thyristors. Algorithm 150 will be described belowin regard to the fabrication of a UMOS insulated-gate bipolar transistorwith the understanding that the description is applicable to other typesof semiconductor devices with minor changes such as the type of implantsor semiconductor layers fabricated.

A “P” well semiconductor layer or anode layer 208 is subsequently formedon a back side 209 of the source layer 204. A current spreadingsemiconductor layer 212 may, in some embodiments, then be formed on aback side 210 of the “P” well layer 204. Subsequently, an “N−” driftlayer 216 is subsequently formed on a back side 214 of the currentspreading layer 212 and a “P+” drain semiconductor layer 220 is formedon a back side 218 of the drifty layer 216. It should be appreciatedthat one or more of the semiconductor layers 204, 208, 212, 216, 220 maybe formed via epitaxially growing the semiconductor layers 204, 208,212, 216, 220 or forming the semiconductor layers 204, 208, 212, 216,220 in an underlying layer via, for example, an ion implantation processor a diffusion process. As illustrated in FIG. 7, the semiconductorlayers 204, 208, 212, 216, 220 are formed on the semiconductor substrate202 in reverse order. That is, once the fabrication of the semiconductordevice 200 is completed, the source layer 204 will be the top-mostsemiconductor layer.

In embodiments wherein the semiconductor layers 204, 208, 212, 216, 220are sequentially epitaxially grown on the substrate 202, the probabilityof the occurrence of lattice damage in the semiconductor device 200 maybe reduced. Lattice damage to the crystal structure of the semiconductordevice 200 may occur during, for example, ion implantation, interruptedepitaxial growth of semiconductor layers (i.e., wherein thesemiconductor device 200 is moved from one fabrication reactor toanother fabrication reactor during the fabrication processes), and thelike.

Referring back to FIG. 5, once the semiconductor layers 204, 208, 212,216, 220 have been formed in process step 154, the semiconductorsubstrate and a portion of the source layer 204 are removed in processstep 156. For example, as illustrated in FIG. 7, the “N+” semiconductorsubstrate 202 and a portion of the “N+” source semiconductor layer 204may be removed as illustrated by the arrow 222. The substrate 202 andthe source layer 204 may be removed by use of, for example, a chemicalmechanical polish process. However, because the source layer 204 isrelatively thin (e.g., about 10 to about 20 micrometers), it may bedifficult to determine the precise amount of thickness removed from thesource layer 204. Accordingly, as illustrated in FIG. 5, in process step158, the thickness of the top semiconductor layer is determined. Notethat after the substrate is removed in process step 156, the sourcelayer 204 is the top-most semiconductor layer of the semiconductordevice 200 (i.e., the “P+” drain layer 220 is the bottom-mostsemiconductor layer).

To do so, a trench length method measure-and-etch sequence may beperformed. For example, as illustrated in FIG. 6, the process step 156may include a first sub-step 170 in which a deep trench 232 is formed ina test area 230 of the semiconductor wafer upon which the semiconductordevice 200 is being formed. As illustrated in FIG. 8 a, the deep trench232 may delineate the test area 230. As illustrated in FIG. 8 b, thedeep trench 232 is illustratively etched to the depth of the currentspreading layer 212. Subsequently, in sub-step 172, a shallow trench 234is etch in the test area 230 to a plurality of known depths using anumber of conductive mask 236 such as, for example, a nickel (Ni)material. As such, the conductive masks 236 are separated by varyingdistances of the source layer 204. Once the shallow trenches 234 havebeen formed, the sheet resistivity between each conductive mask 236 isdetermined in process step 176. Subsequently, in process step 178, aplot 250 may be generated of the conductivity (i.e., the inverse of thesheet resistivity) of the source layer 204 versus the etching depths ofthe shallow trenches 234. The plot 250 has an abscissa axis 252graduated in units of known etching depth of the shallow trenches 232between the conductive masks 236 and an ordinate axis 254 graduated inunits of conductivity between the conductive masks 236. A plot line 256is generated based on the sheet resistivity measurements performed inprocess step 176. The thickness 238 of the source layer 204 may then bedetermined by extrapolating the plot line 256 to the point at which theconductivity reaches a value of zero (i.e., the point 258 at which theplot line 256 intercepts the abscissa axis 252).

Referring back to FIG. 5, once the thickness 238 of the source layer 204has been determined in process step 158, the thickness of the sourcelayer 204 is reduced to a known thickness in process step 160. In theillustrative embodiment, the source layer 204 is etched to a thicknessof about two to about three micro meters. The source layer 204 may be soetched using any suitable etching process such as, for example, areaction ion etch process. Because the thickness 238 of the source layer204 is known prior to the etching step, the source layer 204 may beselectively etched to a known thickness using such etching processes.

Once the source layer 204 has been etched to the desired thickness inprocess step 160, the fabrication of the semiconductor device 200 iscompleted in process steps 162 and 164. Because the illustrativesemiconductor device 200 is a UMOS insulated-gate bipolar transistor, inprocess step 162 a gate trench is formed in the source layer 204, the“P” well layer 208, and the current spreading layer 212. The processstep 162 may be substantially similar to the sub-step 106 of processstep 20 of algorithm 10 described above in regard to FIG. 3.Subsequently, in process step 164, the gate dielectric, gate electrode,source electrodes, and drain electrode are formed on the semiconductordevice 200 in process step 164. Again, the process step 164 may besubstantially similar to sub-step 108 of process step 20 of algorithm 10described above in regard to FIG. 3. As such, the semiconductor 200,once fabricated, may be substantially similar to the semiconductordevice 110 illustrated in and described above in regard to FIG. 4 d.

Referring now to FIG. 10, in another embodiment, an algorithm 300 forfabricating a semiconductor device 350, such as an insulated-gatebipolar transistor device, begins with a process step 302. In processstep 302, a semiconductor substrate 352 is formed as illustrated in FIG.11. Process step 352 is substantially similar to the process step 12 ofalgorithm 12 and the semiconductor substrate 352 may be substantiallysimilar to the semiconductor substrate 52 formed in process step 12. Forexample, the semiconductor substrate 352 may be formed fromsilicon-carbide. It should be appreciated that because the substrates52, 352 are subsequently removed during the fabrication of thesemiconductor devices 50, 350, the substrates 52, 352 may be doped withany type of impurity. For example, the semiconductor 202 may be dopedwith N-type impurities to an “N+” concentration or with P-typeimpurities to a “P+” concentration.

Subsequently, in process step 304, additional semiconductor layers areformed on a front side 354 of the semiconductor substrate 352. Asillustrated in FIG. 11, the semiconductor layers are formed in abottom-to-top sequence. That is, the semiconductor layer that will bethe bottom-most semiconductor layer when the semiconductor device 350 iscompleted is formed first on the front side 354 of the substrate 352.For example, in embodiments wherein the semiconductor device 350 is aUMOS insulated-gate bipolar transistor, a “P+” drain semiconductor layeror anode layer 356 is formed on the front side 354 of the semiconductorsubstrate 352. Similarly, in embodiments wherein the semiconductordevice 350 is a DMOS insulated-gate bipolar transistor, a “P+” drainsemiconductor layer or anode layer is formed on the front side 354 ofthe semiconductor substrate 352. A “N−” drift semiconductor layer 360 isthen formed on a front side 358 of the drain layer 356. Subsequently, insome embodiments, a current spreading semiconductor layer 364 may beformed on a front side 362 of the drift layer 360 and a “P” wellsemiconductor layer or base region 368 is formed on a front side 366 ofthe current spreading layer 364. It should be appreciated that one ormore of the semiconductor layers 352, 356, 360, 364, 368 may be formedvia epitaxially growing the semiconductor layers 352, 356, 360, 364, 368or forming the semiconductor layers 352, 356, 360, 364, 368 in anunderlying layer via, for example, an ion implantation process or adiffusion process. As illustrated in FIG. 11, the semiconductor layers352, 356, 360, 364, 368 are formed on the semiconductor substrate 202 insequential order. That is, once the fabrication of the semiconductordevice 200 is completed, the “P” well layer 368 will be the top-mostsemiconductor layer. In embodiments wherein the semiconductor layers352, 356, 360, 364, 368 are sequentially epitaxially grown on thesubstrate 352, the probability of the occurrence of lattice damage inthe semiconductor device 200 may be reduced. However, because

Referring back to FIG. 10, once the semiconductor layers 352, 356, 360,364, 368 have been formed in process step 304, the semiconductorsubstrate 352 and a portion of the drain layer 356 are removed inprocess step 306. For example, as illustrated in FIG. 1, the “N+”semiconductor substrate 352 and a portion of the “P+” drainsemiconductor layer 356 may be removed as illustrated by the arrow 374.The substrate 352 and the drain layer 356 may be removed by use of, forexample, a chemical mechanical polish process. However, similar to thesource layer 204 of the semiconductor device 200 of FIG. 7, because thedrain layer 356 is relatively thin (e.g., about 10 to about 20micrometers), it may be difficult to determine the precise amount ofthickness removed from the drain layer 356 during the etching process.Accordingly, in process step 158, the thickness of the bottomsemiconductor layer (i.e., the drain layer 356) is determined. Note thatafter the substrate is removed in process step 306, the drain layer 356is the bottom-most semiconductor layer of the semiconductor device 350(i.e., the “P” well layer 368 is the top-most semiconductor layer of thesemiconductor device 350).

To do so, a trench length method measure-and-etch sequence may beperformed. The trench length method measure-and-etch sequence issubstantially similar to the trench length method measure-and-etchsequence performed in and discussed above in regard to the algorithm 158of FIG. 6. Once the thickness of the drain layer 356 has been determinedin process step 308, the thickness of the drain layer 356 is reduced toa known thickness in process step 310. In the illustrative embodiment,the drain layer 204 is etched to a thickness of about two to about threemicrometers. The drain layer 356 may be so etched using any suitableetching process such as, for example, a reaction ion etch process.Because the thickness of the drain layer 356 is known prior to theetching step, the drain layer 356 may be selectively etched to a knownthickness using such etching processes.

Once the drain layer 356 has been etched to the desired thickness inprocess step 310, the fabrication of the semiconductor device 350 iscompleted in process steps 312, 314, and 316. In process step 312, “N+”source regions are implanted in the “P” well layer 368. The “N+” sourceregions may be so implanted by, for example, an ion implantationprocess. Because the illustrative semiconductor device 350 is a UMOSinsulated-gate bipolar transistor, in process step 314 a gate trench isformed in the “P” well layer 208 and the current spreading layer 212.The process step 314 is substantially similar to the sub-step 106 ofprocess step 20 of algorithm 10 described above in regard to FIG. 3.Subsequently, in process step 316, the gate oxide, gate electrode,source electrodes, and drain electrode are formed on the semiconductordevice 350. Again, the process step 350 is substantially similar tosub-step 108 of process step 20 of algorithm 10 described above inregard to FIG. 3. As such, the semiconductor 200 once fabricated issubstantially similar to the semiconductor device 110 illustrated in anddescribed above in regard to FIG. 4 d.

Again, it should be appreciated that the algorithm 300 may be used tofabricate a number of different semiconductor devices including, but notlimited to, UMOS IGBTs, DMOS IGBTs, and thyristors. Although algorithm300 has been described above in regard to a UMOS IGBT, it should beappreciated that the algorithm 300 may be used, with slightmodification, to fabricate other types of high power semiconductordevices. As such, the algorithm 300 is not limited to the fabrication ofUMOS insulated-gate bipolar transistors.

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, such an illustration and descriptionis to be considered as exemplary and not restrictive in character, itbeing understood that only illustrative embodiments have been shown anddescribed and that all changes and modifications that come within thespirit of the disclosure are desired to be protected.

There are a plurality of advantages of the present disclosure arisingfrom the various features of the methods and semiconductor devicesdescribed herein. It will be noted that alternative embodiments of themethods and semiconductor devices of the present disclosure may notinclude all of the features described yet still benefit from at leastsome of the advantages of such features. Those of ordinary skill in theart may readily devise their own implementations of the methods andsemiconductor devices that incorporate one or more of the features ofthe present invention and fall within the spirit and scope of thepresent disclosure as defined by the appended claims.

1. A method for fabricating a semiconductor device, the methodcomprising: forming a silicon-carbide substrate; epitaxially growing afirst semiconductor layer on a front side of the silicon-carbidesubstrate such that the first semiconductor layer is lattice-matched tothe silicon-carbide substrate; and removing the entire silicon-carbidesubstrate after the formation of the first semiconductor layer, whereinthe first semiconductor layer is a drift semiconductor layer of thesemiconductor device having a first concentration of first typeimpurities that is less than a second concentration of first typeimpurities of the silicon-carbide substrate.
 2. The method of claim 1,wherein the semiconductor device is an insulated-gate bipolartransistor.
 3. The method of claim 2, wherein the semiconductor deviceis a DMOS insulated-gate bipolar transistor.
 4. The method of claim 2,wherein the semiconductor device is a UMOS insulated-gate bipolartransistor.
 5. The method of claim 1, wherein epitaxially growing thefirst semiconductor layer on the front side of the silicon-carbidesubstrate comprises epitaxially growing the first semiconductor layer ona silicon side of the silicon-carbide substrate.
 6. The method of claim1, wherein epitaxially growing the first semiconductor layer on thefront side of the silicon-carbide substrate comprises epitaxiallygrowing the first semiconductor layer on a carbon side of thesilicon-carbide substrate.
 7. The method of claim 1, wherein the firstsemiconductor layer has a thickness of about one micrometer.
 8. Themethod of claim 1, wherein removing the entire silicon-carbide substratecomprises using a chemical mechanical polishing process.
 9. The methodof claim 1, further comprising epitaxially growing a secondsemiconductor layer on a front side of the first semiconductor layersuch that the second semiconductor layer is lattice-matched to the firstsemiconductor layer, prior to removing the entire silicon-carbidesubstrate.
 10. The method of claim 1, further comprising epitaxiallygrowing a second semiconductor layer on a back side of the firstsemiconductor layer such that the second semiconductor layer islattice-matched to the first semiconductor layer, after removing theentire silicon-carbide substrate.
 11. The method of claim 1, wherein thefirst semiconductor layer forms a bottom semiconductor layer of thesemiconductor device after the entire silicon-carbide substrate has beenremoved.
 12. The method of claim 1, wherein the first semiconductorlayer forms a top semiconductor layer of the semiconductor device afterthe entire silicon-carbide substrate has been removed.
 13. The method ofclaim 1, wherein removing the entire silicon-carbide substrate comprisesremoving a portion of the first semiconductor layer.
 14. The method ofclaim 13, further comprising epitaxially growing a second semiconductorlayer on a front side of the first semiconductor layer such that thesecond semiconductor layer is lattice-matched to the first semiconductorlayer, prior to removing the entire silicon-carbide substrate and aportion of the first semiconductor layer.
 15. The method of claim 13,further comprising determining a thickness of the first semiconductorlayer subsequent to removing the entire silicon-carbide substrate and aportion of the first semiconductor layer.
 16. The method of claim 15,wherein determining a thickness of the first semiconductor layercomprises determining a thickness of the first semiconductor layer usinga trench length measure technique.
 17. The method of claim 15, whereindetermining a thickness of the first semiconductor layer comprisesdetermining a thickness of the first semiconductor layer based on asheet resistivity of the first semiconductor layer.
 18. The method ofclaim 15, further comprising etching the first semiconductor layer to aknown thickness based on the determined thickness.